Non-porous polycarbonate films less than 1 μm thick

ABSTRACT

Non-porous, thin, polycarbonate films and a process for their production. Solutions of polycarbonates in concentrations of less than 5 parts solids per 100 parts solvent are applied to the surface of liquids immiscible, or sparingly miscible with, the solvent for the polycarbonate. The solvent is then evaporated, and the resultant film removed. The films are useful for the separation of gas and liquid mixtures and for electrical insulating purposes.

This is a division, of application Ser. No. 402,430, filed Oct. 1, 1973now U.S. Pat. No. 3,975,225.

BACKGROUND OF THE INVENTION

Thin, non-porous polymer films are acquiring increasing importance asmembranes in permeation installations for the selective separation ofcertain gases or liquids from gas mixtures or liquid mixtures; inreverse osmosis for desalination of brackish water and seawater; incoatings, for example of metallic workpieces; as electrically insulatingfilms in electrical installations and as dielectrics in condenserconstruction.

Polycarbonate films possess particular importance in these fieldsbecause of their unusual properties. For example, because of their verygood electrical insulating capacity and high heat distortion point theyare used for insulating purposes in electrical plant construction(electric motors, transformers and others) and because of their verygood dielectric properties they are used as dielectrics in condenserconstruction. Because of their high selective permeability they are alsoproposed for the separation of helium from natural gas and because oftheir high permeability for oxygen as against nitrogen they are proposedfor the enrichment of oxygen in atmospheric air.

To increase the yield of permeation installations, to reduce theconstructional size of electrical machinery and to increase the capacityof condensers, it is desirable to manufacture polycarbonate films whichare as thin as possible while at the same time preserving thenon-porosity, and to use these films for the abovementioned purposes.

The reason for this is that the gas permeability or liquid permeabilityincreases in inverse proportion to the film thickness, that is to say areduction of the layer thickness of a film to 1/100 of the originallayer thickness produces a 100-fold increase in the permeability of thefilm. Equally, the capacity of condensers increases in inverseproportion to the layer thickness of the dielectric; a reduction in thelayer thickness of a film to 1/100 of its original layer thickness thusleads to a 100-fold increase in the capacity of the condenser.

The lowest thickness of commercially manufactured polycarbonate films iscurrently 2 μm = 2 × 10⁻ ³ mm (sold under the registered trademarkMAKROFOL of Bayer AG). Such films are manufactured by castingpolycarbonate solutions through slit dies, optionally with subsequentmonoaxial, biaxial or surface stretching.

For the abovementioned end uses it is desirable, as indicated above, tomanufacture non-porous films based on polycarbonate of which thethickness is approx. one to two powers of ten below the currentlyachievable minimum thickness of 2 μm, that is to say between 0.01 and0.2 μm. At these extremely low film thicknesses it will in most cases benecessary to use suitable supporting membranes to ensure the requisitemechanical stability and to mount the polycarbonate films, free oftension, on such carriers. Using the abovementioned currently customarymanufacturing processes, however, it is not possible to manufacture therequisite extremely thin polycarbonate films so that they are non-porousand to mount them, if desired, free of tension on carriers.

SUMMARY AND DETAILED DESCRIPTION

It has now been found that, surprisingly, non-porous films ofthicknesses less than 1 μm can be manufactured from aromaticpolycarbonates if dilute solutions of polycarbonates are applied to thesurface of liquids which are immiscible or sparingly miscible with thesolvents of the polycarbonates, the polycarbonate solutions are allowedto spread and the solvent is evaporated. The solids concentration of thepolycarbonate solutions should be less than or equal to 5 parts per 100parts of solvent. It is particularly advantageous if the solventutilized has as low a surface tension relative to the spreading liquidas possible in order to achieve particularly good spreading of thesolution and hence the formation of uniform polycarbonate films whichare as thin as possible.

It is also advantageous to use solvents of density less than that ofwater.

Furthermore, it is of particular advantage if the spreading liquid,preferably water, is enriched in, or saturated with, the solventutilized and/or if the air space above the spreading liquid is enrichedin, or saturated with, the vapor of the solvent used.

It is to be understood that the polycarbonate solution may be applied tothe surface of the spreading liquid by any suitable means including forexample, pouring, dripping or by means of a slit die.

The solvent may be evaporated by any convenient means including forexample heating the spreading liquid which as the polycarbonate solutionon the surface thereof or by the application of a vacuum.

In a particular embodiment, the polycarbonate solutions can also beapplied to the smooth surface of a water-soluble polymer films or to thesurface of a solid body which optionally carries a water-solubleintermediate layer and the polymer film formed after spreading of thepolymer solution and evaporation of the solvent removed, wherenecessary, by treatment with water, for example by immersion of thecoated polymer film or solid body in water.

If the manufacture of very thin, non-porous films of polycarbonates isto be carried out continously or discontinuously with large surfacedimensions, it is also desirable to apply the polycarbonate solutionfrom a slit die onto the surface of the spreading liquid or onto thesurface of the solid body which is to be coated, and in that case therelative speed between the slit die and the spreading liquid or thesolid body to be coated as well as the concentration of thepolycarbonate solution must be suitably chosen to obtain non-porouspolycarbonate films of the desired thickness.

If the very thin, non-porous polycarbonate film formed is intended to beapplied, free of tension, onto a porous, mechanically resistantsupporting layer or onto other surfaces of solid bodies, this supportinglayer or onto other sufaces of solid bodies, this supporting layer orthe solid body to be coated can be introduced into the spreading liquidand a polycarbonate film that is subsequently formed by casting thepolymer solution on the liquid surface or a polycarbonate film that hasbeen separately manufactured and applied to the liquid surface, may beapplied free of tension onto this supporting layer or onto the solidbody by carefully lowering the level of the liquid.

The aromatic polycarbonates suitable for the manufacture of thenon-porous films of thicknesses less than 1 μm generally have molecularweights M.sub. w greater than 20,000. They may be manufactured frombisphenols according to known processes. A series of bisphenols whichare suitable for the manufacture of such aromatic polycarbonates arecontained in the following Patent Specifications: U.S. Pat. Nos.3,028,365; 2,999,835; 3,148,172; 3,271,368; 2,991,273; 3,271,367;3,280,078; 3,014,891; 2,999,846, the disclosures of which areincorporated herein by reference, DOS (German Published Specification)1,570,703, DOS 2,063,050 and DOS 2,063,052.

Examples of suitable solvents are aliphatic chlorinated hydrocarbons andaromatics, e.g., sym-tetrachloroethane; methylene chloride;cis-1,2-dichloroethylene; 1,2-dichloroethane; styrene and toluene. Anexample of a particularly suitable polycarbonate solution for themanufacture of non-porous films of thicknesses less than 1 μm is asolution of polycarbonate from2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane in styrene.

The polycarbonates from 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propanemay be prepared according to the methods described in DOS 2,063,050 andDOS 2,063,052 and U.S. Pat. No. 3,879,348. According to the processesdescribed in these references tetramethyl-substituted bisphenolpolycarbonates are prepared by reacting the bischlorocarbonic acidesters of the corresponding bisphenols or by reacting the correspondingbisphenols with phosgene or with the bischlorocarbonic acid esters ofthe bisphenols in accordance with the phase boundary condensationprocess, in the presence of aqueous alkali and a solvent suitable forpolycarbonates, through the addition of at least 10 mol percent and upto 200 mol percent, relative to the bisphenol, of tertiary amine. Thesubsequent reaction time is chosen so that the product of the amount ofamine (mol percent) and the reaction time (hours) exceeds a value of 15.The process may be carried out in either one step or several steps. Forexample, the bisphenols are dissolved in aqueous alkali, preferably insodium hydroxide solution or potassium hydroxide solution, and asuitable solvent for the polycarbonate being produced is added. Suitablesolvents of this nature are generally chlorinated hydrocarbons, such asmethylene chloride, chloroform and 1,2-dichloroethane, and alsochlorinated aromatics, such as chlorobenzene, dichlorobenzene andchlorotoluene. Phosgene is passed into this mixture with vigorousstirring. In the case of bisphenols which, because of their hydrophobiccharacter, do not produce bisphenolate solutions, a suspension isadvantageously employed. The amount of phosgene required depends on thebisphenol employed, the stirring action and the reaction temperature,which can lie between about 10° C. and about 60° C., and is in general1.1-3.0 mols of phosgene per mol of bisphenol. After the phosgenation,which can also be carried out in the presence of chain stoppers, forexample 2,6-dimethylphenol, the condensation to give a high molecularpolycarbonate is carried out by adding the tertiary amine, for exampletrimethylamine, triethylamine, dimethylbenzylamine ortriethylenediamine, as the catalyst. The amounts of amine are in general10-200 mol percent, relative to bisphenols, but preferably 10-50 molpercent are employed.

The polycarbonates manufactured in the manner described above may beisolated according to known processes, for example by separating off theaqueous phase, repeatedly washing the organic phase with water until itis free of electrolyte, and thereafter precipitating the polycarbonateor evaporating off the solvent.

The spreading liquid which as stated previously is preferably water is aliquid that is immiscible, or sparingly miscible, with the particularsolvent employed in the polycarbonate solution. Suitable spreadingliquids may be readily determined by those skilled in the art from anychemical text.

liquid very thin, non-porous films of polycarbonates, optionally appliedto a mechanically resistant, porous supporting layer, are used, interalia for the separation of gas mixtures and liqud mixtures. This use ismade possible by the different permeability, intrinsic to thepolycarbonate, towards the various gases, vapors of liquids. Known usesare for the separation of helium from natural gas or for the enrichmentof oxygen in atmospheric air because of the approximately 5-fold greaterpermeability of oxygen as against nitrogen. Since, additionally,polycarbonate has a particularly high permeability for carbon dioxidegas (ratio of the gas permeabilities of CO₂ : O₂ : N₂ = approx. 30:5:1), very thin non-porous polycarbonate films are preferentially suitablefor use as a material for oxygen tents in the clinical field, and asbreathing masks. Such breathing masks can be used if air of low oxygencontent, or heavily polluted air, is to be enriched in oxygen orenriched in oxygen with simultaneous retention of dust-like impuritiesand almost complete retention of harmful gases and vapors (say carexhaust gases, putrefaction gases and others), by virtue of theintrinsic very low permeability of these gases and vapors throughpolycarbonate films. As an example, it may be mentioned that thepermeabilities of oxygen and propane are in the ratio of approx. 35:1.

A further use of these very thin, non-porous polycarbonate films is forelectrical insulating purposes (electrical plant construction, forexample high temperature-resistant electrical insulation for electricmotors, generators, transformers and others) and for dielectricpurposes, for example for electrical condensers. Here it will in mostcases be desirable to apply the polycarbonate films directly onto thematerial to be coated.

The invention may be fully understood by referring to the examples thatfollow.

EXAMPLE 1

Manufacture of polycarbonate from2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane and its bischlorocarbonicacid ester.

22.4 g of sodium hydroxide (0.56 mol) and 22.7 g of2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane (0.08 mol) are dissolvedin 600 ml of water. 100 ml of methylene chloride and 3.0 ml oftriethylamine (0.02 mol) are then added while stirring. 49.1 g of thebischlorocarbonic acid ester of2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane (0.12 mol) dissolved in500 ml of methylene chloride, are added all at once to the mixture,while stirring vigorously. The mixture is then vigorously stirred for afurther 21/2 hours. The process is carried out under nitrogen at 20°-25°C. After the subsequent stirring the batch is worked up. The aqueousphase is free of bisphenol. The organic phase is diluted with methylenechloride and is then washed twice with 5% strength aqueous hydrochloricacid and thereafter water until free of electrolyte. The polycarbonateis precipitated from the organic phase to yield 59 g of a whiteflocculent polycarbonate, the methylene chloride solution of whichyields a clear, tough, strong film. The relative viscosity of thepolymer is 1.529 (in methylene chloride at 25° C., c = 5 g/l). Theaverage molecular weight by light-scattering M_(w) is 83,000 and theglass transition temperature is 206° C.

EXAMPLE 2

A 1% strength solution of2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane-polycarbonate ofmolecular weight M_(w) = 37,000 prepared by methods analogous to Example1, in styrene, was dripped onto water. After spreading the solution andevaporating the solvent, a uniform, completely non-porous polycarbonatefilm formed, the thickness of which, estimated from the interferencecolor, was approx. 0.1 μm. By carefully lowering the water surface, thefilm was applied, free of tension, onto an electrolytically producedcopper grid of mesh width 45 μm located in the water. The non-porosityof the film was checked under an electron microscope at very highmagnification (250,000:1 ).

EXAMPLE 3

A 1% strength solution of2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane-polycarbonate ofmolecular weight M_(w) approx. 100,000, in styrene, was applied to aglass surface rubbed with curd soap; thereafter the glass plate was setup vertically in order to ensure that the solution ran off uniformly andhence a uniform film was formed. After drying, the film was lifted offby immersing the glass plate in water. Here again, the non-porosity waschecked under an electron microscope, analogously to Example 2.

While the invention has been described with reference to certainspecific embodiments thereof, it is not intended to be limited to these.Accordingly, it is intended to embrace all such alternatives,modifications, and variations as fall within the spirit and scope of theappended claims.

What is claimed is:
 1. Non-porous polycarbonate films based on aromaticpolycarbonates from 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propanehaving molecular weights, M_(w), greater than 20,000 and havingthicknesses less than 1 μm.